German Patent Publication (DE-OS) 3,717,427, corresponding to U.S. Ser. No.: 07/196,328, now U.S. Pat. No. 4,836,024, discloses an impact sensor for motor vehicles or the like. The earlier device includes two acceleration sensors, each cooperating with two acceleration signal evaluating circuits. The two evaluating circuits forming a pair have each a different threshold value which constitute first and second reference values. Both reference values must be reached to achieve a trigger signal. The acceleration sensors are directional sensors and are so arranged that each sensor, or rather its sensing axis is directed at a different angle relative to the movement direction of the vehicle. The outputs of two circuit groups are connected to AND gates for a signal combination and the outputs of the two AND gates are connected to an OR gate for producing the trigger signal.
The above mentioned circuit is quite satisfactory for its intended purpose, but it cannot distinguish between rear impacts and lateral impacts on the one hand and front or slanted impacts on the other hand. The known circuit provides a trigger signal in response to a frontal or frontally slanted impact because only in that instance the two lower threshold values are also exceeded in both circuit groups, or rather channels. On the other hand, when a rear impact or lateral impact is involved, the lower threshold values or at least one lower threshold value in one channel is not exceeded so that in that instance the safety device is not triggered. Thus, the ability of the known impact sensor to distinguish between different types of impacts is limited. However, the known sensor is quite adaptable to different structural requirements of different types of vehicles, especially with regard to the collapsible or so-called crumple zone of a vehicle.